This invention relates to ddetermining the concentration of a light hydrocarbon in a relatively high boiling liquid mixture. In one of its aspects this invention relates to the absolute vapor pressure attributable to the adiabatic expansion of a light hydrocarbon in a mixture in its relationship to the relative amount of light hydrocarbon in the mixture. In another of its aspects this invention relates to a mechanical apparatus suitable for increasing the internal volume of a closed chamber. In yet another of its aspects this invention relates to the control of the heat input to a distillation process in response to the continuous sampling and determination of the relative amount of light hydrocarbon within sequential samples taken from the distillation process.
In one of its concepts this invention relates to an apparatus and method for measuring the absolute pressure within a chamber upon expansion of a liquid sample containing light hydrocarbon in a relatively high boiling liquid mixture with the variation in pressure between sequential samples being used as control for the reboiler heat input to the distillation process.
In many extractive distillation or liquid-liquid extraction processes a solvent is used to selectively absorb a light hydrocarbon from a mixture of other light hydrocarbons. The light hydrocarbon-rich solvent is then distilled to remove the desired light hydrocarbon and the lean solvent is cooled and recycled to the extraction process. The efficiency of the extraction process is greatly impaired if light hydrocarbon is not completely removed from the recycled solvent. Typically, the recycled, lean solvent should contain less than .01 weight percent of the light hydrocarbon.
Some typical extraction processes using the system above are the extraction of butadiene from a stream comprising butadiene and butenes using a solvent comprising one of the following as the primary component: (a) furfural, (b) dimethylformamide, (c) high boiling glycols such as di- and triethylene glycol, (d) ethers of such high boiling glycols and the like. Other extraction processes include separations in which high boiling compounds such as esters, ethers, sulfolane, and similar compounds are employed as solvents.
The extraction of butadiene, butene-1 butene-2, n-butane, isobutane, isoprene, propane and the like from mixtures comprising these compounds using appropriate solvents are examples of the systems to which my procedure is applicable. The extraction of aromatics such as benzene, toluene, etc., from paraffins is another example of suitable extraction systems.
Efforts have been made in the past to control the removal of a light hydrocarbon such as butadiene from a solvent such as a mixture of furfural, oils and water, by using temperature control of the lower part of the distillation column. Results have been unreliable because non-ideal distillation is encountered, with the distillation non-idealities changing in an unpredictable manner with pressure, temperature, and concentration ratios. Because of these non-idealities, tray temperature is not a reliable guide for controlling the light hydrocarbon content of the solvent. Lacking an adequate method of measurement and control it has been common practice to overstrip, i.e., to remove light hydrocarbon more completely from the solvent than is warranted by the economics of the process. This overstripping wastes both solvent and energy. In many processes the application of excess heat can cause polymerization of the solvent thereby losing some of the solvent and possibly damaging equipment by polymer laydown.
It is, therefore, an object of this invention to provide an apparatus and method for process measurement and control in which a sample stream is withdrawn from a non-linear distillation process, such as the stripping of butadiene from a solvent mixture of furfural, oils and water, to be tested for absolute vapor pressure at a temperature lower than existing at the sample point. It is a further object of the invention to provide apparatus and method for automatically and continuously measuring the absolute vapor pressure of the liquid sample stream and applying the results obtained in sequential sampling to control of the process from which the samples are withdrawn.
Other aspects and objects of this invention will become apparent upon reading the disclosure and the appended claims.